The present invention relates to a process for stripping photoresist from a substrate. More specifically, the invention relates to processes for removing a layer of photoresist after an ion implantation process is performed.
Fabrication of integrated circuits includes numerous processes. Ion implantation is one such process commonly used in the manufacture of integrated circuits wherein dopant ions are selectively implanted through an organic photoresist mask into a surface of the semiconductor substrate. The organic photoresist mask, typically cast from solvent, is patterned using photolithography processes. Once the ion implantation process is complete, the photoresist mask is typically removed by a stripping process. However, during the ion implantation process, the dopant ions react with a surface of the photoresist to create what is commonly referred to by those skilled in the art as a xe2x80x9ccrust portionxe2x80x9d. The resulting crust portion is relatively nonporous and is difficult to subsequently remove using conventional stripping processes. The presence of a crust in the photoresist layer deleteriously affects post ion implantation processing. For instance, it has been found that the crust portion does not allow trapped volatile solvents or low molecular weight polymers within the photoresist layer to escape easily as the wafer temperature is raised during dry stripping processes. As a result, plasma stripping of the photoresist layer may cause blistering or popping at elevated temperatures, which in turn causes defects and particles to be generated. The term xe2x80x9celevated temperaturesxe2x80x9d refers to those temperatures greater than the maximum temperature the photoresist layer had previously been baked or subjected to prior to the implantation process. Stripping ion implanted photoresist using conventional wet chemical strippers requires longer throughputs since the dissolution behavior of the crust portion requires longer contact time with the strippers. Moreover, the dissolution behavior of the crust portion is not very uniform.
Various methods have been disclosed in the art for removing photoresist layers from substrates that have been exposed to an ion implantation process. For example, Fujimura et al., in U.S. Pat. No. 4,938,839, describe a method that employs cooling the substrate while the ion implanted patterned photoresist layer is stripped from the substrate with a plasma of an etchant gas comprising oxygen. Cooling the wafer during stripping decreases the photoresist stripping rate thereby resulting in longer throughput.
Multiple and complicated plasma process steps have also been attempted but require careful monitoring to determine removal of the crust portion of the photoresist layer. In U.S. Pat. No. 5,811,358 to Tseng et al., the inventors disclose a three step method for stripping photoresist after high dose ion implantation. The crust portion of the ion implanted photoresist is first stripped with an oxygen and nitrogen/hydrogen plasma at a low temperature ( less than 220xc2x0 C.) to minimize or prevent the above noted blistering and/or popping problem. Then, a higher temperature ( greater than 220xc2x0 C.) is employed to remove the remaining photoresist. Finally, the substrate is then cleaned with ammonium hydroxide and hydrogen peroxide to remove remaining contaminant and photoresist residues. This type of multi-step process requires different modules and equipment sets thereby decreasing wafer throughput and increases manufacturing costs.
Kuo et al., in U.S. Pat. No. 6,024,887, disclose a process for stripping ion implanted photoresist by first exposing the ion implanted resist layer with a first plasma generated from oxygen and fluorine gases. A second plasma, without fluorine containing species, is used to strip the ion implanted photoresist. Again, careful monitoring is required to determine removal of the crust layer.
In view of the prior art, it is desired to have a simple stripping process for removing the photoresist mask layer after exposure to a post ion implantation process. Preferably, the stripping process for the ion implanted photoresist requires minimal monitoring, is amenable to high throughput processing and is adaptable to both wet and dry stripping processes.
The present invention overcomes the prior art problems for removing a layer of photoresist after an ion implantation process is performed. The inventive processes have in common the step of subjecting a substrate having an ion implanted photoresist layer thereon to an ultraviolet radiation (UV) exposure. The step of exposing the substrate to UV radiation is after ion implantation and prior to photoresist removal from the substrate. The ion implanted photoresist layer is subsequently stripped or removed using conventional wet or dry stripping processes. In the case of dry stripping, the inventors have surprisingly found that blistering and/or popping problem noted in the prior art is minimized and advantageously, that higher stripping temperatures can be used for the dry stripping processes. In the case of wet stripping, removal efficiency is increased.
The inventive processes for removing ion implanted photoresist includes a step of exposing a substrate having an ion implanted photoresist layer thereon to an ultraviolet radiation source sufficient to render said photoresist layer removable and stripping the photoresist from the substrate. The ultraviolet radiation source preferably emits radiation at a wavelength from between about 150 nm to about 450 nm. More preferably, the ultraviolet radiation source emits radiation at a wavelength from between about 200 nm to about 400 nm. The substrate is exposed to an exposure energy dose of ultraviolet radiation from about 10 mJ/cm2 to about 100 J/cm2. Preferably, the exposure energy dose at the substrate surface is at least 100 mJ/cm2. The time for exposing the substrate is variable and dependent on, among others, the ultraviolet exposure source. Optionally, the substrate may be heated during UV exposure to increase a rate for removing the photoresist mask from the substrate. In the event the substrate is heated, the temperatures used should not be greater than the maximum temperature the photoresist layer had been baked prior to the step of exposure to ultraviolet radiation. Preferably, the temperature range is from about 20xc2x0 C. to about 120xc2x0 C.
The inventive process can be practiced in in a separate module having therein the UV exposure source or in the same chamber as the plasma stripping process that has been adapted to have a UV exposure source.
Other embodiments of the invention are contemplated to provide particular features and structural variants of the basic elements. The specific embodiments referred to as well as possible variations and the various features and advantages of the invention will become better understood when considered in connection with the detailed description and drawings that follows.